Observed mass transports accompanying upstream orographic blocking during MAP IOP8

A mass budget within a domain of 200 km×300 km encompassing the western end of the Po river valley in northern Italy is performed to examine the relative contributions of the multiple sources of moist airflow in the formation of precipitation for a case of strong upstream blocking observed during Intensive Observing Period eight (IOP8) of the Mesoscale Alpine Programme (MAP). Results of this mass budget, made possible by horizontally extensive Doppler radar observations covering this upstream region, reveal that the bulk of the blocked easterly flow undergoes widespread (i.e. mesoscale) slow ascent over a region of relatively flat terrain, and is ultimately lifted to a level at which it can apparently be entrained into the overriding (unblocked) southerly component flow. As a result, the occurrence of profound orographic blocking did not preclude a portion of this low-level air from ultimately surmounting the high Alpine crest. The contribution to this broad mesoscale circulation of smaller-scale downslope flows observed within multiple deep Alpine river drainages is also investigated, by computing observed mass transports emerging from the mouths of several valleys surrounding the Lago Maggiore region. Overall, the down-valley flow is shown to be a relatively minor contributor to the low-level mesoscale transports within the Po basin, but, nonetheless, one which is important to obtain mass balance in these budget computations. Copyright © 2006 Royal Meteorological Society